Carbon dioxide (CO2) accounts for about 76% of global greenhouse gas emissions from human activities, with methane (16%), nitrous oxide (6%), and fluorinated gases (2%) accounting for the balance (United States Environmental Protection Agency). The majority of CO2 comes from the burning fossil fuels to produce energy, although industrial and forestry practices also emit CO2 into the atmosphere. Reduction of greenhouse gas emissions, particularly CO2 emissions, is critical to halt the progression of global warming and the accompanying shifts in climate and weather.
It has long been recognized that catalytic processes may be used to convert gases containing carbon dioxide (CO2), carbon monoxide (CO), and/or hydrogen (H2), such as industrial waste gas or syngas, into a variety of fuels and chemicals. Recently, however, gas fermentation has emerged as an alternative platform for the biological fixation of such gases. In particular, C1-fixing microorganisms have been demonstrated to convert gases containing CO2, CO, and/or H2 into products such as ethanol and 2,3-butanediol. Efficient production of such products may be limited, for example, by slow microbial growth, limited gas uptake, sensitivity to toxins, or diversion of carbon substrates into undesired byproducts. Accordingly, there remains a need for genetically engineered microorganisms having improved characteristics, especially improved carbon fixation, product production, and toxin resistance.